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Re: Loop optimizer issues
- From: Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>
- To: Diego Novillo <dnovillo at redhat dot com>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>,Richard Henderson <rth at redhat dot com>, Jason Merrill <jason at redhat dot com>,pop at gauvain dot u-strasbg dot fr, Jan Hubicka <jh at suse dot cz>,Daniel Berlin <dberlin at dberlin dot org>
- Date: Tue, 29 Jul 2003 20:26:24 +0200
- Subject: Re: Loop optimizer issues
- References: <20030530183552.GA27110@atrey.karlin.mff.cuni.cz> <1054585449.9789.146.camel@frodo.toronto.redhat.com> <20030727224601.GA5476@atrey.karlin.mff.cuni.cz> <1059492808.3164.61.camel@frodo.toronto.redhat.com>
Hello,
> I've been thinking about this proposal and I am not convinced that it
> would be a good idea to merge the two branches.
>
> * The tree-ssa branch is primarily trying to replace RTL passes.
> This may include the loop optimizer. So far, it is not clear
> whether we will actually need to have a sophisticated loop
> optimizer at the RTL level (if at all). The only thing that
> comes to mind that may needed from RTL are:
> * instruction lengths/costs to guide the unroller
> * prefetching capabilities and memory costs to guide
> software pipelining and memory hierarchy optimizations.
Exactly because this question is not clear, there is a need to
experiment. This can hardly be done if the changes are kept separated.
> * While there still isn't much loop-related work on the branch,
> the loop discovery infrastructure that we inherited from
> mainline seems adequate for the basic things.
There is a lot of more things I hope to reuse, especially the analysis
stuff I have written. Yes, it is possible to take just the pieces I
need and I will do it if the other reasons won't persuade you.
> * The branch is slowly becoming increasingly demanding wrt new
> contributions and optimization work. Nightly tests on many
> platforms are very helpful. I don't really want to add anything
> that's not related to the tree optimization infrastructure and
> may or may not be needed.
I propose the changes to be disabled by default (although I of course
would be grateful to people who would enable them for testing).
> * The tree-ssa branch is already very difficult to merge with
> mainline. I don't want to make it more difficult. I would
> prefer to see the rtlopt branch merged into mainline separately.
Which will make the merge even more complicated, IMO.
Zdenek